Aquatic fouling organisms such as barnacles, Tubeworms, common mussels, Bugula neritina, sea squirts, green layer, sea lettuce, and slimes attach to ships (especially the ship bottoms), fishing tools such as fishing nets and fishing net accessories, and structures submerged in seawater such as power plant aqueducts, leading to dysfunction, impaired appearances, and other problems of the ships and so on.
Conventionally, the attachment of aquatic fouling organisms has been heretofore prevented by coating surfaces of ships, fishing tools, and submerged structures with antifouling coating materials that include organic tin-containing copolymers. For example, a coating film formed by application of an antifouling coating material that contains a polymer with tributyltin groups gradually dissolves the polymer component into seawater, causing constant replacement of coating film surfaces, which enables the prevention of the attachment of aquatic fouling organisms to the coating film. After dissolution, the coating film can be reapplied to exhibit continued antifouling effect. However, the use of these antifouling coating materials has now been abandoned due to problems with marine pollution.
Triorganosilyl ester-containing copolymers with triorganosilyl groups, which are less toxic and less environmentally harmful than organic tin groups, have recently been developed and used as hydrolyzable copolymers to replace organic tin-containing copolymers (Patent Documents 1 to 12). However, although a coating film that includes a triorganosilyl ester-containing copolymer initially dissolves in seawater at a constant rate, the dissolving rate of the coating film gradually increases, and becomes excessively high after a long period of time has elapsed, making the design of the coating material difficult. For this reason, an attempt has been made to adjust the dissolving rate of the coating film by using triorganosilyl ester-containing copolymers in combination with rosin (or rosin derivatives) (Patent Documents 1 to 3).
However, when rosin (or a rosin derivatives) is used, although a portion of the rosin reacts with a metal compound contained in the coating composition to form a metal salt during the manufacture of a coating material, the reactivity is insufficient, resulting in the rosin (or rosin derivatives) with free carboxylic acid remaining in the coating composition. The rosin (or rosin derivatives) has high hydrophilic properties, and hence tends to deteriorate the water resistance of the coating film. If the coating film has poor water resistance, it tends to develop defects such as blisters, cracks, and the like.
Accordingly, a method has been suggested to remove the rosin with unreacted free carboxylic acid by pre-mixing an excess of a metal compound with rosin (Patent Document 11). In this method, however, even though an excess of a metal compound is used, the metal compound and rosin do not react sufficiently, making it difficult to completely remove the rosin (or rosin derivatives) having free carboxylic acid.
On the other hand, when a copolymer obtained by the copolymerization of silyl monomers having straight-chain alkyl groups, e.g., tri-n-butylsilyl methacrylate, is used as the above-mentioned triorganosilyl copolymer, the hydrolysis rate of the coating film is very high (i.e., has poor water resistance), making it difficult to control the dissolving rate of the coating film. For this reason, copolymers obtained by the copolymerization of silyl monomers wherein all of the alkyl groups are branched, e.g., triisopropylsilyl methacrylate, are now widely used (Patent Documents 4 to 12). When these copolymers are used, however, the coating films are brittle, and may be cracked, peeled, and the like.
When the above-mentioned copolymer containing triisopropylsilyl methacrylate or a metal (except copper) salt of rosin or rosin derivatives is used, the coating film exhibits stable solubility at a low seawater temperature, i.e., 25 deg. C. or less; however, as the seawater temperature becomes higher, the solubility of the coating film significantly increases, resulting in an unexpectedly large amount of the dissolved coating film. Therefore, it has been difficult to design the coating film thickness when applying an antifouling coating composition to ships that travel into tropical sea areas.    Patent Document 1: JP-A-Hei 10 (1998)-30071    Patent Document 2: JP-A-Hei 11 (1999)-116857    Patent Document 3: JP-A-Hei 11 (1999)-116858    Patent Document 4: JP-A-2000-248029    Patent Document 5: JP-A-2000-248228    Patent Document 6: JP-A-2000-265107    Patent Document 7: JP-A-2001-81147    Patent Document 8: JP-A-2002-53796    Patent Document 9: JP-A-2002-53797    Patent Document 10: JP-A-2002-97406    Patent Document 11: JP-A-2003-261816    Patent Document 12: JP-A-2005-082725